Dielectric solution containing a polycarbonate-polydimethylsiloxane block copolymer additive

ABSTRACT

A dielectric solution with good wetting properties is disclosed as well as a dielectric material comprised of a polyolefin impregnated with the dielectric solution. The solution is comprised of a dielectric liquid and a block copolymer additive which is surface-active therein. The block copolymer is comprised of at least two polymerized comonomers, one of which is soluble in the dielectric liquid thereby distributing the block copolymer therein, and the second of which is sufficiently surface-active in the solution to produce a solution having a surface tension less than that of the dielectric liquid alone.

United States Patent Gaines, Jr. et al.

[ 51 May 23,1972

[54] DIELECTRIC SOLUTION CONTAINING A POLYCARBONATE- POLYDIMETHYLSILOXANE BLOCK COPOLYMER ADDITIVE [72] Inventors: George L. Gaines, Jr., Scotia; Donald G.

Le Grand, Burnt Hills, both of NY.

[73] Assignee: General Electric Company l22| Filed: July 7, 1969 [2| 1 Appl. No.: 839,642

OTHER PUBLlCATlONS Bascom et al., Wetting of Low-Energy Surfaces By Nonaqueous Solutions or Dimethylsiloxane-Containing or fluorocarbon-Containing Surfactants" l. & E. C. Product Research and Development, Vol. 8, No. 2 (June. 1969). pp. 1 l8- 123.

Primary Examiner-John T. Goolkasian Assistant Examiner-Robert A. Dawson Attorney-Charles T. Watts, Paul A. Frank, Jane M. Binkowski, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman AISTRACT A dielectric solution with good wetting properties is disclosed as well as a dielectric material comprised of a polyolefin impregnated with the dielectric solution. The solution is comprised of a dielectric liquid and a block copolymer additive which is surface-active therein. The block copolymer is comprised of at least two polymerized comonomers, one of which is soluble in the dielectric liquid thereby distributing the block copolymer therein, and the second of which is sufficiently surface-active in the solution to produce a solution having a surface tension less than that of the dielectric liquid alone.

6 Claims, No Drawings DIELECTRIC SOLUTION CONTAINING A POLYCARBONATE-POLYDIMETHYLSILOXANE BLOCK COPOLYMER ADDITIVE The present invention relates to dielectric materials. More specifically, it relates to a dielectric solution for electrical apparatus such as capacitors. It also relates to dielectric materials comprising a polymer ordinarily in the form of a film, impregnated with the present dielectric solution.

l-lalogenated aromatic compounds, known as askarels in the industry, and specifically chlorinated diphenyl and related compounds, are widely used as dielectric media in electrical apparatus such as electrical transfonner for cooling and insulating purposes, these compounds having the advantage over mineral oil dielectrics of being nonflammable.

Since many of the electrical characteristics of a dielectric are a result of surface phenomena, it is desirable that a liquid halogenated aromatic dielectric with low surface tension be employed. The lower the surface tension of the dielectric, the greater is its wetting of the electrical components to give better cooling, impregnation, and greater voltage-gradient uniformity. There are, however, a number of disadvantages inherent in the use of wetting agents in liquid halogenated aromatic dielectrics. One such disadvantage is the high incompatibility or insolubility of most wetting agents in dielectrics, so that any distribution thereof in the liquid is a transient condition with the agent soon separating out and concentrating in isolated areas of the surface of the liquid. In such condition, it does not increase the dielectrics wetting properties. Silicone oil, for example, has a low surface tension but when it is admixed with a liquid halogenated aromatic dielectric, it quickly separates out as a separate phase due to the incompatibility of the materials and is not effective in reducing the surface tension of the liquid. Another disadvantage of a number of wetting agents is that they volatilize or decompose under usual conditions of use of the dielectric. Such another disadvantage is that some wetting agents tend to deteriorate the dielectric properties of the liquid.

It is an object of the present invention to produce a halogenated aromatic dielectric solution having substantially uniform low surface tension and increased wetting properties which are attained without significantly diminishing the dielectric properties of the liquid.

Briefly stated, the process of the present invention comprises substantially dissolving a surface-active block copolymer additive in a halogenated aromatic dielectric liquid to produce a solution having a surface tension less than that of the dielectric liquid alone, i.e. at least 2 dynes per centimeter less. The block copolymer additive contains at least two polymerized comonomers.. One of the polymerized comonomer components is soluble in the dielectric liquid and serves to distribute the additive in the liquid. The second polymerized comonomer component is sufficiently surfaceactive in the dielectric liquid to produce a solution having a surface tension less than that of the dielectric liquid alone.

A preferred halogenated aromatic dielectric liquid according to our invention is a compound having from one to five halogen substituents such as chlorine, and from one to three aryl groups. More specifically, a preferred liquid is trichlorodiphenyl which is commercially available as Pyranol 1499. This material has a high corona start and corona extinction voltage.

As used herein, the term surface-active block copolymer additive means one which when dissolved in the halogenated aromatic dielectric liquid depresses the surface tension of the liquid.

The block copolymer additive contains at least two polymerized comonomers. One of the polymerized comonomer components is soluble in the dielectric liquid so that it serves to distribute the additive in the liquid. An example of such a polymerized comonomer is polycarbonate for a dielectric such as trichlorodiphenyl. The second polymerized comonomer component is sufficiently surface-active in the dielectric liquid solution so that the solution has a surface tension less than that of the dielectric liquid alone. Typical examples of such surface activity-imparting comonomers are organosiloxanes such as dirnethylsiloxane, and fluoro-containing monomers such as pentadecafluoro-octyl methacrylate and 2- (N-propylperfluoro-octane sulfonamido) ethyl acrylate. Specifically, polycarbonate-polydimethylsiloxane block copolymers, polystyrene-polydimetl'iylsiloxane block copolymers, and polyphenylene oxide-polydimethylsiloxane block copolymers are suitable as additives for a dielectric liquid such as trichlorodiphenyl to form the solution of the present invention.

The block copolymer additive used in the present invention can vary in composition and structure. It may be a graft, branched, or linear block copolymer. Since it is a block copolymer, it contains repeating segments or blocks of the same monomeric unit and these blocks have a length close to the average block length for that particular monomer. However, the average block length of one comonomer component can differ from the average block length of a second comonomer component and such differing average block lengths largely detemiine the properties of the resulting block copolymer.

An example of a linear block copolymer of A and B monomeric units may have the following structure:

...AAAAABBBAAAABBBBAAAAAABB... where the A block has an average of five units and the B block averages three units. An example of a graft block copolymer of A and B monomeric units may have the following structure:

...AAAAAA...

WWWUJ COUJWU As is well known in the art, such block copolymers are distinguishable from random copolymers which do not contain substantially uniform blocks, e.g. copolymers which on the average contain no particular repeating sequence of the same monomeric units and which may have combinations of units varying widely in length and dispersion (e.g., ABAAAABABBAABBBABAB...

Random copolymers, therefore, are not useful in the present invention since, as a practical matter, they cannot be prepared to have the predetermined essential properties of the present block copolymers.

A block copolymer additive of the present invention can be prepared by a number of conventional techniques. It should be prepared, however, so that it is sufiiciently soluble in the dielectric liquid that it does not significantly diminish dielectric properties and also is sufficiently surface-active in the solution to depress surface tension. ln most instances, the polymerized comonomer component of the block copolymer additive which is soluble in the dielectric liquid and thus promotes distribution of the additive therein should have an average block length of at least three monomer units, and the polymerized comonomer component of the additive which provides the desired surface-activity should have an average block length of at least five monomer units. The maximum average block length of each comonomer component is determinable empirically, i.e. it depends largely on the amount of the block copolymer additive used with a particular dielectric liquid.

The number average molecular weight of the block copolymer additive can vary over a wide range as long as the additive is substantially soluble in the dielectric liquid and is sufi'iciently surface-active therein, i.e. it should not be so high as to make the additive substantially insoluble in the dielectric. However, it should not be so low as to volatilize during operation.

The amount of the block copolymer additive used may be detemtined empirically and depends largely on its solubility and the decrease in surface tension it effects in the dielectric liquid. However, the block copolymer additive should not be used in an amount which would leave sufficient insoluble residue to significantly deteriorate the dielectric properties of the halogenated aromatic liquid. The greater the amount of surface-active component in the block copolymer additive, the less soluble is the additive in the dielectric liquid. Generally, the block copolymer additive should be used in an amount less than 1 percent by weight of the liquid because amounts in excess of 1 percent might alter its dielectric properties although more than 1 percent of the block copolymer can be used if it is sufficiently soluble in the dielectric. On the other hand, a particular block copolymer additive should not be used in an amount so small that the additive cannot get to the surface because of its small diffusion rate.

The solution of the present invention is particularly useful as dielectric media in electrical apparatus such as electrical capacitors for cooling and insulating purposes. The use of a halogenated aromatic dielectric liquid in a capacitor is disclosed in U.S. Pat. No. 3,242,402 assigned to the same assignee as the present invention and incorporated herein by reference. If desired, mixtures of the present halogenated aromatic dielectric solution may be used as well as mixtures of the halogenated dielectric solution or solutions with mineral oil, silicone oil, etc.

In U.S. Pat. No. 3,363,156, assigned to the same assignee as the present invention and incorporated herein by reference, there is disclosed a dielectric material comprised of a polyolefin impregnated with a halogenated aromatic dielectric compound. ONe such material is polypropylene film impregnated with trichlorodiphenyl sold under the trademark Pyranol 1499 and disclosed as a useful dielectric in capacitors. The patent also discloses that such impregnated polyolefins are useful for many electrical applications including insulated electrical devices generally, such as electrical cable and transformers.

Specifically, the patent discloses the solid polyolefin resin dielectric material as having numerous minute discontinuities otherwise characterized as apertures, voids, and interstices, whose presence is a recognized characteristic of the resin. This material is described as nonporous in that there are few if any pores or passages interconnecting and passing through side surfaces which would permit the movement of an impregnated fluid through opposite side surfaces. The patent teaches that the polyolefin resin material is impregnated with a dielectric liquid impregnant which permeates the material itself as well as filling these discontinuities, and the composite constitutes a continuous, through heterogeneous, dielectric system. Surprisingly, the kind of impregnation obtained by these teachings in combination with specific materials leads to a kind of synergistic effect the result of which increases the dielectric strength of the combination. In one respect impregnation increases the electrical insulating qualities of the resin dielectric by incorporating in the resin an impregnant material having a higher dielectric strength than the air or gas in the described discontinuities.

One embodiment of the present invention is a dielectric material as disclosed in U.S. Pat. No. 3,363,156 except that the impregnating liquid is the dielectric solution of the present invention.

We have found that the present solution, for example a solution of Pyranol 1499 and a proper block copolymer additive, has wetting properties superior to that of Pyranol 1499 alone. The present solution, therefore, would permeate a film of the polyolefin faster and more completely. Such ease of permeation is especially important in capacitors utilizing tightly wound rolls. It is also important in that the impregnant can reach heretofore difficult to reach voids and interstices in remote areas from origin of impregnation.

In carrying out the process of the present invention, the polyolefin can be impregnated by a number of conventional techniques. Prior to impregnation, the polyolefin is preferably shaped by a conventional process into a desired form which depends on its final application and generally it is in the shape of a film. There is a relationship between the kind and degree of impregnation and the dielectric properties of the impregnated polyolefin material. When the voids and interstices of the polyolefin are essentially filled with impregnating solution and the impregnation process includes both absorption of the impregnant solution by the polyolefin itself as well as partial solution of the polyolefin in the impregnating solution, the polyolefin is referred to as essentially or completely impregnated. It is this essentially completely impregnated material which is preferred in the present invention because it possesses the most satisfactory dielectric properties.

As disclosed in U.S. Pat. No. 3,363,156, polyolefins, and particularly polypropylene, may be impregnated to an unexpectedly high degree with halogenated aromatic liquids, and when so impregnated these elements combine nd coact to provide an improved dielectric spacer. A preferred material of this class of polyolefins is polypropylene resin, particularly an isotactic biaxially oriented polypropylene film, a more complete description of one example of which may be found in Applied Plastics, Nov. 1961, pp. 35 through 64, and Modern Dielectric Materials, LB. Birks, London Heywood and Co., 1960, pp. -142, incorporated by reference herewith.

The polyolefin material of the mentioned articles may be described as linear, regular head to tail polymers of unsaturated hydrocarbons of the formula CH =CHR, i.e., alpha olefins, where R is a saturated aliphatic, an alicylic, or an aromatic radical, copolymers of said unsaturated hydrocarbons with one another, and copolymers of the unsaturated hydrocarbons with at least one other monomer copolymerizable therewith.

The invention may also be practiced effectively with other members of the polyolefin family of materials, particularly polyethylene and poly-4-methyl pentene-l. For more specialized applications other combinations may include impregnated crosslinked polyethylene, or paper materials impregnated with the polyolefin of this invention, for example a paper impregnated with a melt or solution containing polypropylene with the resultant material impregnated with a solution of Pyranol dielectric and polycarbonatepolydimethylsiloxane block copolymer additive.

The impregnated material of the present invention is useful as dielectric media in capacitors and for many electrical applications including insulated electrical devices generally, i.e. all the uses set forth in U.S. Pat. No. 3,363,156.

All parts and percentages used herein are by weight unless otherwise noted.

The invention is further illustrated by the following examples.

In the following examples, the tests and materials used were as follows.

Surface tension was measured with a duNouy tensiometer ring. The surface tension is approximate due to the high viscosity of the liquids.

The contact angle between a drop of liquid and a plane solid surface is one method of measuring wettability. When the contact angle is zero the liquid is considered to wet the solid completely. The contact angle increases with decreasing wettability, and on a solid having a uniform surface, the contact angle is independent of the volume of the drop. In measuring contact angles, it is noted that there is a difference between the contact angle observed when a liquid boundary advances for the first time over a dry, clean, smooth surfaced and the contact angle observed when the liquid boundary recedes from the previously wetted surface. It is the advancing contact angle measured herein and this advancing contact angle was determined by profile view measurements of the contact angle between the drop of liquid and the film surface using a protractor on an eyepiece fitted to a low-power telescope. The film surface was extruded polypropylene sheet sold under the trademark Profax 6520.

The polycarbonate-polydimethylsiloxane block copolymer was prepared substantially as set forth in U.S. pat. No. 3,189,662. The polycarbonate was the intercondensation product of 2,2-bis(4-Hydroxyphenyl) propane and phosgene.

The particular composition and average block length of the block copolymer was determined by the method of preparation and by nuclear magnetic resonance spectroscopy.

EXAMPLE 1 In this example two dielectric solutions of the present invention were prepared and their properties determined and compared to that of the dielectric liquid alone.

One dielectric solution of the present invention was prepared by admixing a polycarbonate-polydimethylsiloxane block copolymer with trichlorodiphenyl, sold under the trademark Pyranol 1499, at room temperature to give a solution containing a 1.1 milligrams of the block copolymer per milliliter of solution, and it is labeled solution A in the following table. The block copolymer had a number average molecular weight of 53,300 and was comprised of 75 percent by weight polycarbonate having an average block length of 18.5 monomer units and 25 percent by weight polydimethylsiloxane having an average block length of 20 monomer units.

The second dielectric solution of the present invention was prepared at room temperature by admixing Pyranol 1499 with another polycarbonate-polydimethylsiloxane block copolymer to give a solution containing 0.8 milligram of the block copolymer per milliliter of solution and it is labeled Solution B in the following table. The block copolymer of Solution B had a number average molecular weight of 72,000 and was comprised of 46 percent by weight polycarbonate having an average block length of 7.08 monomer units and 54 percent by weight polydimethylsiloxane having an average block length of 20 monomer units.

The surface tension and advancing contact angle of each of the solutions was determined as well as that of Pyranol 1499 alone and the results are shown in the following table.

Approximate Advancing surface tension contact Composition dynes/cm. Angle Control Pyranol 1499 43.5 53 Solution 1.1 mg/ml solution of 26.6 20

A Pyranol 1499 and block copolymer Solution 0.8 mf/ml solution of 24.7 13

B Pyranol 1499 and block copolymer The above table illustrates the significantly lower surface tension of the solutions of the present invention as compared to that of the dielectric liquid, Pyranol 1499, alone. Since these solutions have such a small amount of block copolymer dissolved therein, their dielectric properties are not significantly affected.

The improved wetting properties of the solutions of the present invention are illustrated by their advancing contact angles in the above table which are significantly lower than that of the Pyranol 1499 alone.

EXAMPLE 2 In this example extruded polypropylene sheet sold under the trademark Profax 6520 was used.

A drop of Pyranol 1499 was placed on a surface of the sheet and spread with a glass rod. It was observed that the Pyranol 1499 tried to reform the drop because of its non-wetting characteristics. The same test was carried out with Solution A and Solution B of Example 1. Each of these solutions remained spread out as a thin film which indicates that their wetting properties are not only greater than that of Pyranol 1499 alone but also that these wetting properties are substantially uniform.

EXAMPLE 3 In this example, the dielectric properties of two dielectric solutions of the present invention were determined and compared to those of the dielectric liquid alone.

Three standard electric cells were used and are herein referred to as Cells A, B and C. Cell A had a capacitance of 29.5182 picofarads and a cell constant, K(T), of 324.7. Cell A was filled with Pyranol 1499 alone and was the control cell.

Cell B had a capacitance of 29.8788 picofarads and a cell constant, K(T), of 382.7. It was filled with a dielectric solution comprised of Pyranol 1499 having dissolved therein 0.1 10 gram of block copolymer per cc. of the solution. The block copolymer had a number average molecular weight of 53,300 and was comprised of 75 percent by weight polycarbonate having an average block length of 18.5 monomer units and 25 percent by weight of polydimethylsiloxane having an average block length of 20 monomer units.

Cell C had a capacitance of 30.1 187 picofarads and a cell constant, I(("'G), of 33 1 .3. This cell was filled with a dielectric solution comprised of Pyranol 1499 having dissolved therein 0.080 gram of a block copolymer per 100 cc. of the solution. The block copolymer had a number average molecular weight of 72,000 and was comprised of 46 percent by weight polycarbonate having an average block length of 7.08 monomer units and 54 percent by weight of polydirnethylsiloxane having an average block length of 20 monomer units. The percent dissipation factor of each of these three cells was determined under the same conditions as shown in Table I.

TABLE I Frequency Percent Dissipation Factor At 23 C. At At 23 C. (Hertz) Initial 100 C. Final Cell A 50 0.01513 0.3912 0.02366 100 0.00746 0.1954 0.01166 1000 0.000742 0.01956 0.001149 5000 0.000190 0003920000265 10,000 0.000 0.00197 0.00021 Cell B 50 0.02876 0.3094 0.01449 100 0.001305 0.1543 0.00740 1000 0.001302 0.01542 0.000746 5000 0.000335 0.003096 0.000250 10,000 0.000290 0.001564 0.000290 Cell C 50 0.01748 0.1762 0.0092

Table I illustrates that there is no significant difference in the percent dissipation factor of Cells A, B and C when it is determined under the same conditions. The small differences in the values in Table I for Cells B and C, as compared to Cell A, are considered to be within experimental error in the art. Table I shows that the dielectric properties of the solutions of the present invention, as illustrated by their use in Cells B and C, are substantially the same as those of the dielectric liquid alone as shown by Cell A.

The resistivity of each of these cells was also determined by applying to each cell 500 volts DC. for one minute. The results are shown in Table 11.

Table II illustrates that there is no significant difference in the resistivity of the three cells when it is determined under the same conditions. The difference in the values of Table II EXAMPLE 4 In this example, the dielectric properties of two dielectric solutions of the present invention were determined and compared to those of the dielectric liquid. Both solutions and the liquid had dissolved therein the same amount, i.e. 0.35 weight percent, of diepoxycyclohexane. This diepoxycyclohexane is commonly used as a scavenger material in halogenated aromatic dielectric liquids to provide electrical capacitors with improved electrical properties.

Three standard electric cells were used and are herein referred to as Cells A, B and C'Cell A had a C. of 30,6309 picofarads and a cell constant, K( T), of346. 129. Cell A was filled with Pyranol 1499 having dissolved therein the diepoxycyclohexane and was the control cell.

Cell B had a capacitance of 30.2354 picofarads and a cell constant, K( 'T), of 341.772. It was filled with a dielectric solution comprised of the Pyranol 1499 having dissolved therein the diepoxycyclohexane and 0.110 gram of block copolymer per 100 cc. of the solution. The block copolymer had a number average molecular weight of 53,300 and was comprised of 75 percent by weight polycarbonate having an average block length of 18.5 monomer units and 25 percent by weight of polydimethylsiloxane having an average block length of 20 monomer units.

Cell C had a capacitance of 28.8487 picofarads and a cell constant, K(T), of 325.990. This cell was filled with a dielectric solution comprised of Pyranol 1499 having dissolved therein the diepoxycyclohexane and 0.080 gram of a block copolymer per 100 cc. of the solution. The block copolymer had a number average molecular Weight of 72,000 and was comprised of 46 percent by weight polycarbonate having an average block length of 7.08 monomer units and 54 percent by weight of polydimethylsiloxane having an average block length of 20 monomer units. The percent dissipation factor of each of these three cells was determined under the same conditions as shown in Table 111.

TABLE III Frequency Percent Dissipation Factor At 23 C. At At 23 C. (Hertz) Initial 100 C. Final Cell A 50 0.04320 0.41 1 0.02838 100 0.0190 0.250 0.01382 1000 0.00192 0.0206 0.001371 5000 0.00042 0.0041 0.000310 1 0,000 0.00028 0.00205 0.000230 Cell B 50 0.05478 0.520 0.02560 100 0.02283 0.260 0.01201 1000 0.002283 0.0260 0.001192 5000 0.00049 0.0052 0.000280 10,000 0.00031 0.00259 0.00022 Cell C' 50 0.0688 0.704 0.03954 100 0.0353 0.352 0.01785 1000 0.003548 0.0352 0.001785 5000 0.000740 0.0070 0.000375 10,000 0.00044 0.0055 0.00027 TAble lll illustrates that there is no significant difference in the percent dissipation factor of Cells A, B and C when it is determined under the same conditions. The small difl'erences in the values in Table Ill for Cells B and C as compared to Cell A, are considered to be within experimental error in the art. Specifically, Table III shows that the dielectric properties of the solutions of the present invention containing a scavenger material, as illustrated by their use in Cells B and C, substantially the same as those of the dielectric liquid alone with the same scavenger material, as shown by Cell A, and therefore, the present solutions have no significant effect on the scavenger material.

The resistivity of each of these cells was also determined by applying to each cell 500 volts DC. for one minute. The

results are shown in Table IV.

Table IV illustrates that there is no significant difference in the resistivity of the three cells when it is determined under the same conditions. The difference in the values of Table IV for Cells B and C', as compared to Cell A'are considered to be within experimental error in the art. Table IV further shows that the dielectric properties of the solutions of the present invention containing a scavenger material are substantially the same as those of the dielectric liquid alone with the same scavenger material, and therefore, the present solutions have no significant effect on the scavenger material.

What is claimed is:

l. A dielectric solution with increased wetting properties and having a substantially uniform surface tension comprising a halogenated aromatic dielectric liquid having substantially dissolved therein a block copolymer additive which is surfaceactive in said liquid, said block copolymer additive being comprised of a first polymerized aromatic comonomer and a second polymerized organosiloxane comonomer, said first polymerized comonomer having an average block length of at least three monomer units being substantially soluble in said dielectric liquid and effective to distribute said additive therein, and said second polymerized comonomer having an average block length of at least five monomer units being sufficiently surfaceactive in said liquid to depress its surface tension at least 2 dynes per centimeter, said additive being a polycarbonate-polydimethylsiloxane block copolymer and said dielectric liquid being comprised of an aromatic compound having from one to five halogen substituents and from one to three aryl groups.

2. A dielectric solution according to claim 1 wherein said halogen substituents are chlorine.

3. A dielectric solution according to claim 2 wherein said dielectric liquid is trichlorodiphenyl.

4. A dielectric solution according to claim 1 wherein said block copolymer additive is present in an amount ranging from about 0.001 to 1 percent by weight of the dielectric liquid.

5. A dielectric product comprising a polyolefin impregnated with the dielectric solution of claim 1.

6. A dielectric product according to claim 5 wherein said polyolefin is polypropylene. 

2. A dielectric solution according to claim 1 wherein said halogen substituents are chlorine.
 3. A dielectric solution according to claim 2 wherein said dielectric liquid is trichlorodiphenyl.
 4. A dielectric solution according to claim 1 wherein said block copolymer additive is present in an amount ranging from about 0.001 to 1 percent by weight of the dielectric liquid.
 5. A dielectric product comprising a polyolefin impregnated with the dielectric solution of claim
 1. 6. A dielectric product according to claim 5 wherein said polyolefin is polypropylene. 